Two approaches have heretofore been used to provide position information directly at the same position as magnetic data along a track of a magnetic recording disk. These involve magnetic buried servo information and capacitively buried servo information. The magnetic buried servo approach is not practical because of data-to-servo interference. While this interference can be eliminated with azimuth recording (i.e., by recording data at one angle, such as 15.degree., and the servo pattern at another angle, such as -15.degree.), the requisite thin film azimuth recording heads are very difficult to fabricate. The capacitive buried servo approach is relatively complex and costly; and while it is economically feasible for files using large diameter disks providing substantial storage capacities, it is not economical and/or feasible for files using smaller disks.
The most pertinent prior art of which applicants are aware is the unexamined Japanese Patent Application published Mar. 5, 1987 as 62-51022 in the Patent Abstracts of Japan. This application discloses a flexible magnetic recording medium in which concentric grooves are formed in a flexible polyester substrate. Copper pieces are embedded in these grooves by electroless copper plating. Track sector and servo information is encoded by the electrostatic capacitance of the copper pieces; whereupon a magnetic layer overcoat is applied for recording magnetic indicia in superposed relationship over the electrostatic servo pattern. Grooving and then inserting conductive elements in the grooves lithographically requires an undesirable number of fabrication steps. Moreover, the conductive servo pattern elements are not charged. To be sensed, they must be used as capacitive elements in some sort of tuned circuit, such as a pair of sensing electrodes, as the metallic elements of the pattern do not seem to be electrically connected to ground. The sensing circuit must be an active circuit having a high frequency carrier. If track densities of 2,000 tracks per inch are to be attained, radio frequencies in excess of 100 MHz may be required; and in such case, the capacitance of the metallic pattern elements would detune the radio frequency sensing circuit or unbalance a bridge circuit. The modulation envelope of the carrier frequency would then need to be detected, filtered and converted into a position sensing servo signal.
This approach to signal processing is thus very expensive and complicated. In fact, applicants' assignee experimented with a capacitive buried servo approach using patterns similar to those described in this unpublished Japanese application. While it was a technical success, it was never implemented commercially because it greatly and undesirably increased disk file cost.
Of incidental interest is U.S. Pat. No. 4,489,278 which discloses a voltage detecting device that uses an insulating film made of a material such as Teflon* material capable of storing electrostatic charges applied over a conductive layer of a recording medium. However, there is no teaching of superimposing electrostatic charge patterns over, under or in between magnetic indicia. FNT *Trademark of E. I. DuPont deNemours & Company
There is a need for an improved, cost-efficient method of increasing the storage capacity of a magnetic recording medium by use of an improved position sensing system.